The head of NATO warned member countries on Thursday against allowing a wedge to be driven between the United States and Europe, as concern grows about Washington’s commitment to its allies should Donald Trump return to office.

Faced with a war in Ukraine that is draining military and financial resources, and with a U.S. package of support held up by infighting in Congress, European leaders and senior officials have warned that Europe must invest more in its armies and new technologies and ramp up weapons production.

"I welcome that the European allies are investing more in defense, and NATO has called for that for many, many years," NATO Secretary-General Jens Stoltenberg told reporters at the alliance's Brussels headquarters, where he was chairing a meeting of the organization’s defense ministers.

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"But that’s not an alternative to NATO. That is actually a way to strength NATO. And we should not pursue any path that indicates that we are trying to divide Europe from North America," he said.

Talk has even surfaced in recent weeks about Europe developing a nuclear umbrella. France and the United Kingdom – a staunch U.S. ally that sees NATO as the world’s key security organization – are Europe’s only nuclear powers.

France has traditionally seen itself as a counterweight to U.S. influence in NATO. It does not participate in NATO’s nuclear planning group.

"NATO has a nuclear deterrent, and this has worked for decades," Stoltenberg said. "We should not do anything to undermine that. That will only create more uncertainty and more room for miscalculation and misunderstanding."

President Emmanuel Macron insists that France must maintain its independence when it comes to the possible use of nuclear weapons. He said in December, though, that France has a "very special responsibility" as a nuclear power in Europe and "stands by" its allies and European partners.

Talk of a European nuclear umbrella has come from, among others, German members of the European Parliament. But Chancellor OIaf Scholz and other top security policy officials believe there is no alternative to NATO’s nuclear umbrella.

German Defense Minister Boris Pistorius dismissed the debate about European nuclear weapons, saying that it’s a "complex discussion" that shouldn’t be embarked on because of remarks from an aspiring candidate who’s in election campaign mode.

On Saturday, former President Trump, the front-runner for the Republican Party’s nomination this year, said he once warned that he would allow Russia to do whatever it wants to NATO members that are "delinquent" in devoting 2% of GDP to defense.

President Joe Biden branded Trump’s remarks "dangerous" and "un-American," seizing on the former president’s comments as they fuel doubt among U.S. partners about its future dependability on the global stage.

Stoltenberg said those comments call into question the credibility of NATO’s collective security commitment — Article 5 of the organization’s founding treaty, which says that an attack on any member country will be met with a response from all of them.

"The nuclear debate is really the last thing we need at the moment," Pistorius told reporters in Brussels on Wednesday. "It is an escalation in the discussion that we don’t need."

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German Vice Chancellor Robert Habeck also said that "this big abstract debate won’t lead to success." Speaking to Germany’s Welt television, he also voiced skepticism about the idea of making French nuclear weapons part of a European atomic arms strategy.

"The last thing the French want is European co-management over their army," he said.

NATO’s nuclear deterrence relies in part on U.S. warheads deployed in Europe using local infrastructure. A number of NATO countries contribute aircraft for use in a nuclear role, along with trained personnel, but Washington retains ultimate control over the use of these weapons.

NATO conducts a major nuclear exercise every year to ensure its readiness and to act as a deterrent to any would-be aggressor, primarily Russia.